Stimulus overselectivity, also known as restricted stimulus control, is a widely acknowledged problem in the education of individuals with developmental disabilities. Overselectivity refers to learning that is atypically limited with respect to range, breadth, or number of stimuli or stimulus features. In the education of individuals with developmental disabilities, overselectivity may be related to inefficient, incomplete, or inconsistent performance with multi-element stimuli (e.g., printed words). The research will investigate variables that affect overselectivity and the development of stimulus control by multiple or multielement stimuli. From the existing literature on our study population of individuals with moderate to severe developmental disabilities, one may conclude that the number of remedial studies is remarkably small, given the prevalence of the problem. Importantly, no generally applicable approach toward solving the problem has yet emerged. Our preliminary studies have set the stage for such an effort by providing compelling evidence that overselectivity (1) is open to manipulation by reinforcement contingencies, (2) may be directly related to deficiencies in observing behavior, and (3) can be greatly reduced by interventions that control observing behavior. The goals of the proposed research are: (1) To compare the prevalence and degree of overselectivity in children and adolescents who have mental retardation and autism, versus those who have mental retardation only, and to examine the relation between overselectivity and measured mental age (MA) scores. (2) To investigate environmental variables that may affect the behavioral aspects of stimulus overselectivity, including reinforcement variables that have been little studied in past research. (3) To develop effective remedial procedures that reduce or eliminate overselectivity. Approaches include stimulus control shaping procedures (e.g., fading) to shape effective observing, and the use of response shaping procedures to teach participants to make explicit responses that guide effective observing. (4) To examine the generalization of remedial training across tasks in a laboratory study of overselectivity. (5) To examine generalization across tasks and settings in field studies to be conducted in special-education classrooms with tasks that have been identified in participating students' educational plans.